Investigation of Engine Oil Base Stock Effects on Low Speed Pre-Ignition in a Turbocharged Direct Injection SI Engine 2016-01-9071

Low speed pre-ignition (LSPI) is a type of engine knocking specific to turbocharged direct injection engines. There are numerous factors that affect the amount of LSPI produced by an engine. Some of these factors are related to engine hardware; others are specific to the fuel and engine oil. In this paper we focus on the effect of the engine oil base stock on LSPI. We conducted experiments in a stationary engine operating at conditions chosen to accentuate pre-ignition. Our results indicate that base stock viscosity has a statistically significant effect on the amount of low speed pre-ignition. Directionally, we found that engine oils formulated with higher viscosity base stocks produced more pre-ignition events. We measured the effect of base stock quality (defined by either Viscosity Index or concentration of aromatic species) to be small and statistically insignificant in our experimental data. In the course of our experimentation we detected a drift in the LSPI results from our test engines, and instituted an engine-hours correction on our results to compensate. We recommend that other researchers be wary of such a drift.